Dark grains will definitely acidify the mash. Something else to do is to add about 2 tsp per 5 gallons of Lactic Acid to your sparge water. Since a lot of grain buffering gets taken away in the first runnings, acidifying the sparge helps.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Bourbon Barrel Imperial Stout
On Deck: German Lager